Freeman Dyson did the math, and I am not a physicist. I am assuming some sort of mass/energy trade-off. http://en.wikipedia.org/wiki/Project_Orion_(nuclear_pro

Freeman Dyson did the math, and I am not a physicist. I am assuming some sort of mass/energy trade-off. http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Interstellar_missions http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi... He figured 133 years to Alpha Centauri but without carrying extra mass to slow down. I assume you'd have to double that at least if you carried bombs to decelerate.